1. Field of the Invention
This invention relates to new and improved apparatus for restoring the polarization state of light emerging from an optical fiber. In particular, it relates to a new and improved active system for restoring the polarization of light emerging from a single-mode optical fiber link. Accordingly, it is a general object of this invention to provide new and improved apparatus of such character.
2. General Background
The single-mode fibers that are used in 1.2 .mu.m to 1.6 .mu.m optical communication systems nominally have circular symmetry and can transmit two orthogonally polarized light waves with essentially equal propagation constants. When a linearly polarized laser is coupled into such a fiber, the light output at a detector end of the optical fiber likely has a different polarization state due to birefringent effects of small differences in propagation constants of the two fiber modes caused by deviations from core circularity, strains, or other environmental effects.
Such effects as bending, twisting, or mounting are discussed in the literature, including "Linear polarization in birefringent single-mode fibers" by R. H. Stolen, V. Ramaswamy, P. Kaiser, and W. Pleibel, Appl. Phys. Lett. 33 (8) pp. 699-701 (1978) and "Polarization effects in short length, single-mode fibers" by V. Ramaswamy, R. D. Standley, D. Sze, and W. G. French, The Bell System Technical Journal 57 (3) pp. 635-651 (1978).
The change in polarization state may be inconsequencial for some optical receivers. However, when optical processing of light through polarization sensitive devices is desired, the polarization of the light should first be restored to a reference polarization state. Polarization sensitive devices, such as micro-optical thin film electro-optical switches and modulators, are discussed in "Performance limitations imposed on optical waveguide switches and modulators by polarization", R. A. Steinberg and T. G. Giallorenzi, Applied Optics 15 (10) pp. 2440-2453 (1976).
An optical fiber is not necessarily perfectly circular throughout its entire length. Stresses along the fiber are not necessarily completely symmetrical about its central axis. Temperatures can vary at different portions of the fiber, etc., so that if a horizontally polarized light is applied to one end, such horizontally polarized light would not likely be emitted from the opposite end due to birefringence of the fiber. Most likely, the output from the fiber would be elliptically polarized, with varying degrees of ellipticity between linear polarization and circular polarization, and with the major elliptic axis at an arbitrary orientation angle relative to some reference orientation.
Apparatus for restoring the polarization state of light emerging from a single-mode optical fiber should be active, because the fiber link birefringence is subject to slow changes due to mechanical and thermal variations along its length. Previous approaches to restoring the polarization state required that some fraction of the optical signal be diverted from the desired output so that its polarization state could be sensed to provide a feedback signal for controlling the polarization compensating elements. Such approaches are described in "Polarization stabilization on single-mode fiber", R. Ulrich Appl. Phys. Lett. 35(11) pp. 840-842 (1979) and "Electro-optical polarization control on single-mode optical fibres" M. Kubota, T. Oohara, K. Furuya, and Y. Suematsu, Electr. Lett. 16 (15) p. 573 (1980). The polarization compensation has been accomplished using linear birefringence obtained by applying transverse compression to a section of optical fiber by means of two electromagnets or by applying electrical signals to electro-optic elements, transmitting the light after it emerges from the fiber. In both cases, the resulting polarization was sampled using an essentially polarization-insensitive beamsplitter and a set of four photodetectors each fitted with a polarization-sensitive filtering device. Prior art devices utilized, typically, a fixed percentage of output light for determining its polarization state. A sample of its intensity was then returned to the system to control the compensation of the light traveling therethrough.
A common problem with all prior art devices is the finite range of the birefringence that could be produced. This is because, as a birefringent effect of the compensator is varied to compensate for any random variations in the input polarization state, it is possible that a limiting value is reached beyond which it cannot operate. At that point, it has to shut down, requiring restarting. The compensator is set to its mid-range and then varied. During that reset process, the signal likely drops out. The receiving system that requires a linearly polarized output does not get the linearly polarized output. In lieu thereof, a spurious output occurs for a transient period, until the device is reset.